1. Field of the Invention
The present invention relates to a spread spectrum communication system. More specifically, the present invention relates a spread spectrum communication system in which an information signal is communicated in a remote control or remote sensing.
2. Description of the Prior Art
Conventionally, a spread spectrum communication system is known, wherein a carrier signal a spectrum of which is spread by a binary pseudo noise code (hereinafter, simply called as "PN code") having a spectrum width sufficiently broader than an information signal is transmitted, and at a receiving side, an original information signal is restored by multiplying a received signal by a PN code which is the same as that being used at a transmitting side.
In such a spread spectrum communication system, since the spectrum of the information signal is spread by a PN code having a broader spectrum width, in order to correctly restore the information signal, it is necessary to synchronize the PN code which is generated at the receiving side with the PN code at the transmitting side.
As a method for synchronizing both the PN codes, a tau-dither method as shown in FIG. 1 is well known. In FIG. 1, a received spread spectrum signal is inputted to a first multiplier 1a through an input terminal IN. In addition, an output of a VCO (voltage-controlled oscillator) 2 is phase-modulated by an output signal from a low-frequency oscillator 3 such as a multivibrator in a phase demodulator 4. An output from the phase demodulator 4 is given to a PN code generator 5 and used as a clock signal therein. A PN code from the PN code generator 5 is multiplied by the received spread spectrum signal in the first multiplier 1a. An output from the first multiplier 1a is given to a demodulator 8 through a bandpass filter 7a. An output of the bandpass filter 7a is envelope-detected in the demodulator 8 to be outputted to an output terminal OUT. An output of the demodulator 8 is further given to a second multiplier 1b through a bandpass filter 7b. Then, in the second multiplier 1b, the output signal from the low-frequency oscillator 3 is multiplied by an output signal from the bandpass filter 7b. An output signal of the second multiplier 1b is given to the VCO 2 through a low-pass filter 9 as a control signal.
In the spread spectrum communication system, it is known that a level of the output signal from the first multiplier 1a is changed as shown in FIG. 2 in accordance with a relative change of phases of the PN code included an inputted spread spectrum signal and the PN code from the PN code generator 5.
Now, assuming that an initial phase of the PN code from the PN code generator 5 exists a position of a point aa of FIG. 2 and the phase is advanced and shifted to a point ab, a relative phase of both the PN codes plys between both the points aa and ab in accordance with a rectangular signal from the low-frequency oscillator 3, and in response thereto, the output signal of the first multiplier 1a is subjected to an amplitude modulation at the same frequency as that of the rectangular signal. This amplitude-modulated components are derived by the bandpass filter 7b and thereafter multiplied by the rectangular signal in the second multiplier 1b so that a direct current signal having a correct polarity and a correct level is obtainable to be given to the VCO 2 as a control signal. A frequency of the VCO 2 is changed by the control signal, thereby to change the phase of the PN code from PN code generator 5.
In addition, in a case where the relative phase of the both PN codes plys between points ba and bb, a polarity of the output of the first multiplier 1a is reversed and a phase change of the PN code from the PN code generator 5 is also reversed.
Furthermore, in a case where the relative phase of the both the PN codes plys between points ca and cb sandwiching a peak of a correlation output as shown in FIG. 2, no change occurs in an amplitude of the output signal from the first multiplier 1a. Therefore, no amplitude-modulated components to be supplied to the second multiplier 1b exist, and thus, an oscillation frequency of the VCO 2, that is, the phase of the PN code from the PN code generator 5 is not changed.
In such a tau-dither method, in order to synchronize the phase of the PN code at the receiving side with that of the PN code at the transmitting side, it is necessary to use the phase modulator 3 as shown in FIG. 1. Therefore, a circuit configuration thereof was complex.